Only Sindre can conclusively answer this question I think. The only way I see it working is you install Subsonic on to both the computers, edit the vmoptions file to place the subsonic server directory on your external drive (I recently posted on this
here), and stop the Subsonic service whenever the external drive isn't connected.
Because of the windows services that run, I don't think it's easy to be that portable. Unless you use the standalone installer, which I haven't used on a windows machine (only when I was running LAMP). I don't have much experience with Tomcat and Windows, but if you can get a portable version of Tomcat it might work out really well.
Good luck.